An optical device is a device that creates, manipulates, or measures electromagnetic radiation. Optical devices include simple equipment such as lenses, magnifiers and mirrors, as well as more intricate apparatuses such as microscopes, telescopes, lasers, video displays, cameras and specialized imaging and targeting systems. Specialized optical devices such as electro-optic/infrared (EO IR) sensors, laser imaging systems and targeting sensors are often critical components in military and commercial applications involving surveillance, reconnaissance, targeting, analysis and space exploration. Such devices are often required to operate with a high degree of reliability and precision in a variety of different settings including ground, water, airborne and space environments.
Many optical devices are required to efficiently and reliably transmit and/or detect specific types of electromagnetic radiation (or frequency ranges thereof) under varying and often inclement environmental conditions. In order protect vital sensory or transmissive components, such electro-optical devices often include an optical window formed of a material that is transparent for a wavelength range of photons to be transmitted or detected. Optical windows are sometimes coated with materials that protect the optical surface, alter its reflective or transmissive properties, or somehow affect the electromagnetic radiation contacting the optical surface.
An anti-reflective (AR) coating, for example, is a type of optical coating applied to optical surfaces in order to reduce reflection. This can improve the efficiency of an optical device by allowing more photons to pass through the optical window. Reducing reflection can also improve contrast and sensitivity in certain optical devices by reducing the proportion of stray light that passes through the optical window and reaches the photo-sensitive instrument. In simpler applications such as eyeglasses, AR coatings are often applied to lenses to reduce glint and to make the eyeglass user's eyes more visible to others.
FIG. 1 illustrates one example of an optical window 1 including an optical substrate 2 coated with an anti-reflective coating 4 whose surface acts as the optical surface 6 for an optical device.
Optical surfaces may also be coated with protective materials that protect the surface against damage or surface modification (e.g., staining or smudging) that may occur through contact with the environment. For example, electronic devices which include an interactive touch-screen display (e.g., smart phones) are often smudged and/or stained with fingerprints, skin oil, sweat, cosmetics, etc., during use. Once these stains and/or smudges adhere to the optical surface they may not be easily removed and can in some cases cause permanent damage. Optical surface damage can profoundly affect the performance of specialized electro-optic devices—such as military and commercial devices operated in harsh environments. In some cases, optical surface damage or contamination can jeopardize the ability of a specialized device to detect and/or transmit the required electromagnetic wavelengths.
FIG. 2, for example, illustrates optical surface damage than can occur to an optical surface 6 of a field-turreted electro-optic device 3 after prolonged exposure to harsh environmental conditions. The turreted optical instrument 3 of FIG. 2 is from a forward-looking infrared (FLIR) camera in which the optical surface 6 is embedded within an optical turret assembly 8. Exposure of the electro-optic device 3 to heat and various contaminants over an extended period of time has caused both staining (spotting) and corrosion to the AR coating 4.
The optical surface 6 of FIG. 2 contains a region 10 of relatively low surface corrosion and a region 12 of relatively high surface corrosion, which are clearly delineated by the corrosion boundary line 11. High levels of surface corrosion as seen in region 12 can occur via a number of different mechanisms depending upon the composition of the AR coating 4 and the optical substrate 2. For example, when the optical window 1 is composed of a silicon substrate 2 coated with an AR coating 4 of magnesium fluoride (MgF2) having an undercoating or presence of praseodymium (Pr), then an originally-green-tinted optical surface 6 can turn yellow within the region 12 of high surface corrosion due to oxidation of the Pr. Alternatively, for example, when the optical window 1 is composed of a quartz substrate 2 coated with an AR coating 4 of MgF2 or titanium dioxide (TiO2) having an undercoating or presence of hafnium (Hf), then the optical surface 6 turns white or becomes frosted within the region 12 of high surface corrosion due to oxidation of the Hf. The irregular spotting 14 contained on the optical surface 6 can occur due to contact with organic and inorganic contaminants-especially when the contaminated optical surface 6 is exposed to high temperatures.
When such damage occurs, it is typically necessary to replace the optical window 1, which may require the entire optical device or assembly (e.g., the optical turret assembly 8) to be transported back to a manufacturing site, depot, or laboratory for replacement. Both the process of disassembly and the time required for transportation can be very expensive. For specialized military and commercial systems employing such optical devices, disassembly and replacement may render the systems inoperable and unable to accomplish critical missions. In such cases the military or commercial user may therefore be required (as a matter of practicality) to procure and store a spare device or assembly which is both expensive and in some scenarios highly impractical.
Although protective coatings could, in some optical devices, be applied to the surface of an AR coating 4 or an optical substrate 2 in order to reduce or eliminate surface corrosion and spotting, conventional protective coatings cannot be applied to many specialized electro-optics that detect certain electromagnetic frequencies. Some protective coatings can adversely affect the performance of highly-sensitive electro-optic detection and imaging devices due to the optical characteristics of the protective materials or the thickness of the protective coatings.